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Evaluation of microstructural and mechanical properties of AlxCrFeMnNi high entropy alloys
Materials Today: Proceedings xxx (xxxx) xxx
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Evaluation of microstructural and mechanical properties of AlxCrFeMnNi high entropy alloys Khumo Masemola ⇑, Nicholus Malatji, A. Patricia Popoola Department od Chemical, Metallurgical Engineering and Materials Sciences, Tshwane University of Technology, P.M.B X680, Pretoria, South Africa
a r t i c l e
i n f o
Article history: Received 29 September 2019 Received in revised form 16 October 2019 Accepted 26 December 2019 Available online xxxx Keywords: AlxFeNiCrMn High entropy alloys Compressive strength Microhardness and annealing
a b s t r a c t The aim of this work is to investigate microstructural and mechanical properties of the AlxFeNiCrMn HEAs fabricated by arc-melting method followed by annealing at 1000 °C in air. Microstructural features and phase formation of the developed alloy were examined using scanning electron microscope (SEM) and X-ray diffraction (XRD) respectively. The microhardness and compressive strength measurement were performed with a diamond base indenter and Instron compression machine. It was noticed that Al addition encouraged the formation of dendrites in the alloys with dendritic cores and interdendritic regions. The microhardness decreased from 495 HV to 397 HV with increasing Al concentration. Adjusting the Al content above 10 at% increased the ultimate strength from 1200 MPa to 1429 MPa. However, ductility was reduced which significantly lowered the total elongation by around 5%. The alloy with x = 15, displayed a good combination of high strength and ductility due to a reasonable balance in the volume fraction of BCC and FCC solid solution phases. The addition of Al is beneficial to the mechanical performance of the AlxFeNiCrMn alloy, but inadequate amounts will be counterproductive. Ó 2020 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the scientific committee of the 2nd International Conference on Recent Advances in Materials & Manufacturing Technologies.
1. Introduction An emerging class of alloys better known as High Entropy Alloys (HEAs) have received significant scientific interest among researchers. The design concept of these alloys differs from the traditional single or dual principal component alloy design [1–3]. Yeh defined that a HEA consists of a minimum of five major elements in equimolar or near equimolar ratio each with the concentration ranging between 5 and 35 at% [4–6] as well as possess simple solid solution with high mixing entropy [7,8] Owing to their chemical complexity, these alloys are said to exhibit exclusive structures and properties unattainable in traditional alloys. Undeniably, some of the investigated HEAs displayed exceptional properties such as high strength, excellent toughness, good tribological performance and many more [9]. The properties of HEAs are also governed by the alloying elements, elemental composition and fabrication method. The choice of elements used can determine the phase evolution and properties of the HEAs [10]. Generally, fcc HEAs display great ductility and softness, while bcc HEAs have eminent strength
at the expense of low tensile ductility. Hence, FCC alloys require improvement on yield strength and bcc alloys on plasticity. Therefore, numerous attempts in material design have been made to strike a reasonable balance between the ductility and strength in order to expand the application pf HEAs [11]. Heat treatment is one of the most successful ways of producing a unique combination of both good ductility and high strength [12]. Munitza and others [13] studied the influence of heat treatment on the mechanical properties of AlCrFeNiTi0.5 high entropy alloys. It was discovered that heat treatment at a suitable temperature resulted in morphological changes which augmented both ductility and strength. Similarly, Gu and associates [14] investigated the dependence of tensile properties and microstructure of the FeCrCoMnNi alloy on annealing temperature. The authors concluded that optimizing annealing temperatures can regulate the morphology to obtain good amalgamation of high strength and ductility. The aim of this work is to investigate microstructural and mechanical properties of the AlxFeNiCrMn HEAs fabricated by arc-melting method followed by annealing at 1000 °C in air.
⇑ Corresponding author. E-mail address: [email protected] (K. Masemola). https://doi.org/10.1016/j.matpr.2019.12.320 2214-7853/Ó 2020 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the scientific committee of the 2nd International Conference on Recent Advances in Materials & Manufacturing Technologies.
Please cite this article as: K. Masemola, N. Malatji and A. Patricia Popoola, Evaluation of microstructural and mechanical properties of AlxCrFeMnNi high entropy alloys, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.12.320
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K. Masemola et al. / Materials Today: Proceedings xxx (xxxx) xxx
2. Methodology High purity elemental powders of Al, Ni, Fe, Mn and Cr were thoroughly mixed according to the desired composition in a tubular mixer for 8 h. the ad-mixed powder were then arc-melted in argon containing furnace to create an inert atmosphere during the heating. The melting process of the powdered mixture was carried out in copper crucibles producing as-cast buttons with dimensions of 20 mm 10 mm. Resulting ingots were sectioned and annealed at 1000 °C in the presence of air for 2 h. Morphology and phase composition of the heat treated samples were investigated using scanning electron microscope (SEM) and X-ray diffractometer (XRD) with Cu target Ka radiation respectively. Samples for morphological evaluation were prepared using standard metallographic procedures and etched using the Kroll’s solution. Phase identification of the annealed alloys was carried out using the XPert High Score Plus Software. Microhardness measurements were recorded at 6 different points on the surface of the samples at a load of 500 kgf with a retention time of 10 s. lastly, compression tests were carried out using the Instron 1342 machine. The cross section of the samples was 8 8 mm2 with a height of 10 mm. 3. Results and discussion Fig. 1 shows the SEM micrographs of the AlxFeNiCrMn experimental alloys annealed at 1000 °C. it seen that alloy with Al content of 10 at% (Fig. 1(a)) constitutes mostly of spherical
structures with the presence of noodle-like and irregularly shaped precipitates evenly distributed throughout the surface of the alloy. It is important to note that spherical grains accounts more for a large volume of white phase than the black phase. While the noodle-like and irregularly shaped structures comprises mostly of the gray and black phases respectively. No indication of dendritic structures was observed for this alloy. Fig. 1(b and c) on the other hand, reveals the formation of dendritic structure composed of (i) dendritic core (DC) composing of cuboidal precipitates and (ii) interdendritic regions (ID) consisting of columnar structures. It is evident that the increase in aluminium content fosters the formation of dendrites and coarsening of grains. This was also the case in the reported alloys [15]. The formation of DC and ID within the alloy entails that solid-solution phases coexist in the alloy [16]. The addition of Al to 20 at% caused the formation of well pronounced grain boundaries after annealing at 1000 °C. In addition to Fig. 1(b and c), gray and black are representative of main phases since they occupy a larger volume on the surface of the alloys. A significant reduction in the white phase was observed as the Al concentration was increased. The prediction on formation of single phase solid solutions has to meet the proposed criteria where the formation of FCC and BCC develop in the region of X 1.1, d 6.6%, 15 kJ/mol < DHmix < 5 kJ/mol [17]. The VEC criterion is another factor that is to predict the stability of phases. For instance, FCC phase forms when VEC 8.0 and BCC at VEC < 6.87. for two phases to coexist the VEC value has to be between the range 6.87–8.0 [18]. Table 1 contains the cal-
Fig. 1. SEM images of alloys annealed at 1000 °C with Al at% of (a) 10, (b) 15 and (20).
Please cite this article as: K. Masemola, N. Malatji and A. Patricia Popoola, Evaluation of microstructural and mechanical properties of AlxCrFeMnNi high entropy alloys, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.12.320
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Table 1 Empirical values of enthalpy (DHmix) and entropy (DSmix) of mixing, melting point (Tm), atomic size mismatch (d), regular solution-interaction parameter (X) and valence electron concentration (VEC) of the alloys with varying Al content (atm%).
10 15 20
DHmix (kJ/mol) 9.32 10.80 12.48
DSmix (J/Kmol)
Tm (K)
d
X
VEC
13.08 13.31 13.38
1721.88 1678.07 1634.26
5.83 4.76 5.26
2.42 2.06 1.75
7.28 7.03 6.80
culated values of the parameters used to predict the formation of solid solutions. Calculated values show that the conditions favourable for the development of FCC and BCC phases are met. Moreover, sole BCC phase in absence of FCC is expected when x = 20 and a mixture of both phases when x = 15 and 10. These predictions are confirmed by XRD patterns of the experimental alloys. The XRD patterns of the current AlxFeNiMnCr HEAs are shown in Fig. 2. It can be seen that the alloys constitute mainly of bodycentered cubic (BCC) and face-centered (FCC) structures. At x = 10, the sharp peaks of BCC structure depicts the high amount of Cr while the low intensity peaks of FCC indicates the presence of Fe and Ni. The Al10FeNiMnCr alloy displays a larger volume fraction of FCC phase. However, the alloy containing Al content of 15 at % shows a lower volume fraction of FCC and at 20 at% no FCC was identified. It is evident that the increase in Al in the AlxFeNiMnCr HEA decreases the quantity of FCC in the alloys. These results are consistent with what has been reported in literature [9]. The Al10FeNiMnCr alloys displays almost equal distribution of BCC and FCC phase due to a lower amount of Aluminium. Fig. 3 shows the microhardness of the AlxFeNiCrMn high entropy alloys. The increment of Al promoted decrease in the hardness of alloys. With Al content up to 10 at% a comparatively high hardness value of 495 HV was recorded followed by 410 HV with x = 15, and 397 HV at x = 20. This behaviour as reported in other alloys [19], can be attributed to the fact that Al is softer as compared to other elements having higher strength in the alloy. In addition, this confirms that there is no formation of intermetallic phases within the alloys which may harden the alloy. The low hardness value displayed by the equi-atomic HEA may be due to recrystallization caused by excessive annealing temperatures for prolonged periods. Shahmir and others [12] discovered that short term annealing of CoCrFeNiMn alloy at 800 °C for 10 min prevents grain growth and leads to a combination of high strength and good ductility. Compressive properties of the heat treated AlxFeNiCrMn HEAs are shown in Fig. 4. As can be seen, the plasticity of alloys decreases with increasing Al concentration. The highest total elongation of
600 Microhardness (HVN)
Al at% in alloy
495
500
410
397
400
300
10
15 Al at% in alloy
20
Fig. 3. Vickers hardness measurements of heat-treated AlxFeNiMnCr alloys.
around 24% was observed in the alloy containing x = 10. This could be due to the presence of larger volume fraction of FCC phase as compared to alloys consisting x = 15 and x = 20 which is depicted from XRD results. It has been reported that FCC contains more slip directions and therefore readily experiences slip deformation which ultimately lowers the strength and hardness of the material [20]. With Al concentration of x = 10 the ultimate strength is approximately 1200 MPa. However, when x = 15 and x = 20 the obtained ultimate strength was around 1429 and 1222 MPa respectively with similar total elongation of nearly 20%. Adjusting the Al content above 10 at% significantly reduced the plasticity of the alloys. The presence of BCC solid-solution phase exhibits high strength and hardness with poor ductility [21]. The Al15FeNiCrMn displayed a good combination of high strength and ductility which could be attributed to adequate amounts of BCC and FCC phases in the high entropy alloy. Noteworthy, Al addition may improve the mechanical performance of the alloy. However, excessive amounts will be counterproductive. This observation is in agreement with the reported results [22].
Engineering stress (MPa)
1600 1400 1200 1000
x = 20
800
x = 15
600
x = 10
400 200 0
0,00
10,00
20,00
30,00
40,00
Engineering strain (%) Fig. 2. XRD patterns of the annealed AlxFeNiMnCr.
Fig. 4. Compression curves of AlxCrFeNiMn alloys.
Please cite this article as: K. Masemola, N. Malatji and A. Patricia Popoola, Evaluation of microstructural and mechanical properties of AlxCrFeMnNi high entropy alloys, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.12.320
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4. Conclusion The AlxFeNiCrMn HEAs were successfully fabricated via arcmelting and annealed at 1000 °C for 2 h in air. Microstructural evolution, microhardness and compressive strength properties of the alloys under investigation were examined. It is concluded that the increment of Al caused the hardness to decrease in the annealed alloy. An inverse relationship between the compressive strength and total elongation was observed with the addition of Al. An increase in elemental Al fostered a reduction in plasticity and an increase compressive strength. Moreover, the formation of dendritic structures were identified with Al above 10 at%. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements The authors would like to acknowledge the following organizations. Mintek, Advanced Materials Division, Randburg. Tshwane University of Technology, Department of Chemical, Metallurgical Engineering and Materials Sciences, Pretoria, South Africa References [1] A. Munitz, S. Samuh, E. Brosh, S. Salhov, N. Derimow, R. Abbaschian, Liquid phase separation phenomena in Al2.2CrCuFeNi2 HEA, Intermetallics 97 (2018) 77–84. [2] Y. Chen, S. Zhu, X. Wang, B. Yang, G. Han, L. Qiu, Microstructure evolution and strengthening mechanism of Al0.4CoCu0.6NiSix (x¼0e0.2) high entropy alloys prepared by vacuum arc melting and copper injection fast solidification, Vacuum 150 (2018) 84–95. [3] A.J.B. Xi Jin, L. Zhang, Y. Zhou, X. Du, Y. Liang, B. Li, A new CrFeNi2Al eutectic high entropy alloy system with excellent mechanical properties, J. Alloys Compd. 770 (2019) 655–661. [4] Z.G. Zhu, K.H. Ma, X. Yang, C.H. Shek, Annealing effect on the phase stability and mechanical properties of (FeNiCrMn)(100–x)Cox high entropy alloys, J. Alloys Compd. 695 (2017) 2945–2950. [5] C.-C. Yen, H.-N. Lu, M.-H. Tsai, B.-W. Wu, Y.-C. Lo, C.-C. Wang, S.-Y. Chang, S.-K. Yen, Corrosion mechanism of annealed equiatomic AlCoCrFeNi tri-phase highentropy alloy in 0.5 M H2SO4 aerated aqueous solution, Corrosion Sci. (2019).
[6] S.S. Nene, M. Frank, K. Liu, S. Sinha, R.S. Mishra, B.A. McWilliams, K.C. Cho, Corrosion-resistant high entropy alloy with high strength and ductility, Scripta Mater. 166 (2019) 168–172. [7] S. Aristeidakis, I. and M.-I. T. Tzini, High Entropy Alloys. University of Thessaly, 2016. [8] S. Chikumba, V.V. Rao, High entropy alloys: development and applications Pretoria, in: 7th International Conference on Latest Trends in Engineering & Technology, 2015, pp. 13–17. [9] N.D. Stepanov, D.G. Shaysultanov, R.S. Chernichenko, M.A. Tikhonovsky, S.V. Zherebtsov, Effect of Al on structure and mechanical properties of Fe-Mn-CrNi-Al non-equiatomic high entropy alloys with high Fe content, J. Alloys Compd. 770 (2018) 194–203. [10] Rajesh K. Mishra, R.R. Shahi, Effect of annealing conditions and temperatures on phase formation and magnetic behaviour of CrFeMnNiTi high entropy alloy, J. Magn. Magn. Mater. (2018) 169–175. [11] J.M. Parka, J. Moona, J.W. Baea, J. Junga, S. Leea, H.S. Kima, Effect of annealing heat treatment on microstructural evolution and tensile behavior of Al0.5CoCrFeMnNi high-entropy alloy, Mater. Sci. Eng. A 728 (2018) 251–258. [12] H. Shahmir, J. He, Z. Lu, M. Kawasaki, T.G. Langdon, Effect of annealing on mechanical properties of a nano crystalline CoCrFeNiMn high-entropy alloy processed by high-pressuretorsion, Mater. Sci. Eng. A 676 (2016) 294–303. [13] A. Munitza, S. Salhova, G. Guttmanna, N. Derimowb, M. Nahmanya, Heat treatment influence on the microstructure and mechanical properties of AlCrFeNiTi0.5 high entropy alloys, Mater. Sci. Eng. A 742 (2019) 1–14. [14] J. Gu, S. Ni, Y. Liu, M. Song, Regulating the strength and ductility of a cold rolled FeCrCoMnNi highentropy alloy via annealing treatment, Mater. Sci. Eng. A 755 (2019) 289–294. [15] L.J. Zhang, K. Guo, H. Tang, M.D. Zhang, J.T. Fan, P. Cui, Y.M. Ma, P.F. Yu, G. Li, The microstructure and mechanical properties of novel Al-Cr-Fe-Mn-Ni highentropy alloys with trimodal distributions of coherent B2 precipitates, Mater. Sci. Eng. A 757 (2019) 160–171. [16] M. Chen, L. Lan, X. Shi, H. Yang, M. Zhang, J. Qiao, The tribological properties of Al0.6CoCrFeNi high-entropy alloy with the sigma phase precipitation at elevated temperature, J. Alloys Compd. 777 (2019) 180–189. [17] Y. Yin, J. Zhang, Q. Tan, W. Zhuang, N. Mo, M. Bermingham, M.-X. Zhang, Novel cost-effective Fe-based high entropy alloys with balanced strength and ductility, Mater. Des. 162 (2019) 24–33. [18] P. Cui, Y. Ma, L. Zhang, M. Zhang, J. Fan, W. Dong, Yu. Pengfei, G. Li, R. Liu, Effect of Ti on microstructures and mechanical properties of high entropy alloys based on CoFeMnNi system, Mater. Sci. Eng. A 737 (2018) 198–204. [19] Raju, C.V.S., D. Venugopal, P.R. Srikanth, K. Lokeshwaran, M. Srinivas, C.J. Chary, and A.A. Kumar. Effect of aluminum addition on the properties of CoCuFeNiTi high entropy alloys. In: ICAMM. Materials today: Proceedings, 2016. [20] S. Jiang, Z. Lin, H. Xu, Y. Sun, Studies on the microstructure and properties of AlxCoCrFeNiTi1-x high entropy alloys, J. Alloys Compd. 741 (2018) 826–833. [21] Yukun Lv, Hu. Ruyi, Zhihao Yaoa, Jian Chen, Dapeng Xua, Yong Liub, X. Fana, Cooling rate effect on microstructure and mechanical properties of AlxCoCrFeNi high entropy alloys, Mater. Des. 132 (2017) 392–399. [22] J. Jayaraj, P. Thirathipviwat, J. Han, A. Gebert, Microstructure, mechanical and thermal oxidation behavior of AlNbTiZr high entropy alloy, Intermetallics 100 (2018) 9–19.
Please cite this article as: K. Masemola, N. Malatji and A. Patricia Popoola, Evaluation of microstructural and mechanical properties of AlxCrFeMnNi high entropy alloys, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.12.320
Contents lists available at ScienceDirect
Materials Today: Proceedings journal homepage: www.elsevier.com/locate/matpr
Evaluation of microstructural and mechanical properties of AlxCrFeMnNi high entropy alloys Khumo Masemola ⇑, Nicholus Malatji, A. Patricia Popoola Department od Chemical, Metallurgical Engineering and Materials Sciences, Tshwane University of Technology, P.M.B X680, Pretoria, South Africa
a r t i c l e
i n f o
Article history: Received 29 September 2019 Received in revised form 16 October 2019 Accepted 26 December 2019 Available online xxxx Keywords: AlxFeNiCrMn High entropy alloys Compressive strength Microhardness and annealing
a b s t r a c t The aim of this work is to investigate microstructural and mechanical properties of the AlxFeNiCrMn HEAs fabricated by arc-melting method followed by annealing at 1000 °C in air. Microstructural features and phase formation of the developed alloy were examined using scanning electron microscope (SEM) and X-ray diffraction (XRD) respectively. The microhardness and compressive strength measurement were performed with a diamond base indenter and Instron compression machine. It was noticed that Al addition encouraged the formation of dendrites in the alloys with dendritic cores and interdendritic regions. The microhardness decreased from 495 HV to 397 HV with increasing Al concentration. Adjusting the Al content above 10 at% increased the ultimate strength from 1200 MPa to 1429 MPa. However, ductility was reduced which significantly lowered the total elongation by around 5%. The alloy with x = 15, displayed a good combination of high strength and ductility due to a reasonable balance in the volume fraction of BCC and FCC solid solution phases. The addition of Al is beneficial to the mechanical performance of the AlxFeNiCrMn alloy, but inadequate amounts will be counterproductive. Ó 2020 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the scientific committee of the 2nd International Conference on Recent Advances in Materials & Manufacturing Technologies.
1. Introduction An emerging class of alloys better known as High Entropy Alloys (HEAs) have received significant scientific interest among researchers. The design concept of these alloys differs from the traditional single or dual principal component alloy design [1–3]. Yeh defined that a HEA consists of a minimum of five major elements in equimolar or near equimolar ratio each with the concentration ranging between 5 and 35 at% [4–6] as well as possess simple solid solution with high mixing entropy [7,8] Owing to their chemical complexity, these alloys are said to exhibit exclusive structures and properties unattainable in traditional alloys. Undeniably, some of the investigated HEAs displayed exceptional properties such as high strength, excellent toughness, good tribological performance and many more [9]. The properties of HEAs are also governed by the alloying elements, elemental composition and fabrication method. The choice of elements used can determine the phase evolution and properties of the HEAs [10]. Generally, fcc HEAs display great ductility and softness, while bcc HEAs have eminent strength
at the expense of low tensile ductility. Hence, FCC alloys require improvement on yield strength and bcc alloys on plasticity. Therefore, numerous attempts in material design have been made to strike a reasonable balance between the ductility and strength in order to expand the application pf HEAs [11]. Heat treatment is one of the most successful ways of producing a unique combination of both good ductility and high strength [12]. Munitza and others [13] studied the influence of heat treatment on the mechanical properties of AlCrFeNiTi0.5 high entropy alloys. It was discovered that heat treatment at a suitable temperature resulted in morphological changes which augmented both ductility and strength. Similarly, Gu and associates [14] investigated the dependence of tensile properties and microstructure of the FeCrCoMnNi alloy on annealing temperature. The authors concluded that optimizing annealing temperatures can regulate the morphology to obtain good amalgamation of high strength and ductility. The aim of this work is to investigate microstructural and mechanical properties of the AlxFeNiCrMn HEAs fabricated by arc-melting method followed by annealing at 1000 °C in air.
⇑ Corresponding author. E-mail address: [email protected] (K. Masemola). https://doi.org/10.1016/j.matpr.2019.12.320 2214-7853/Ó 2020 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the scientific committee of the 2nd International Conference on Recent Advances in Materials & Manufacturing Technologies.
Please cite this article as: K. Masemola, N. Malatji and A. Patricia Popoola, Evaluation of microstructural and mechanical properties of AlxCrFeMnNi high entropy alloys, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.12.320
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K. Masemola et al. / Materials Today: Proceedings xxx (xxxx) xxx
2. Methodology High purity elemental powders of Al, Ni, Fe, Mn and Cr were thoroughly mixed according to the desired composition in a tubular mixer for 8 h. the ad-mixed powder were then arc-melted in argon containing furnace to create an inert atmosphere during the heating. The melting process of the powdered mixture was carried out in copper crucibles producing as-cast buttons with dimensions of 20 mm 10 mm. Resulting ingots were sectioned and annealed at 1000 °C in the presence of air for 2 h. Morphology and phase composition of the heat treated samples were investigated using scanning electron microscope (SEM) and X-ray diffractometer (XRD) with Cu target Ka radiation respectively. Samples for morphological evaluation were prepared using standard metallographic procedures and etched using the Kroll’s solution. Phase identification of the annealed alloys was carried out using the XPert High Score Plus Software. Microhardness measurements were recorded at 6 different points on the surface of the samples at a load of 500 kgf with a retention time of 10 s. lastly, compression tests were carried out using the Instron 1342 machine. The cross section of the samples was 8 8 mm2 with a height of 10 mm. 3. Results and discussion Fig. 1 shows the SEM micrographs of the AlxFeNiCrMn experimental alloys annealed at 1000 °C. it seen that alloy with Al content of 10 at% (Fig. 1(a)) constitutes mostly of spherical
structures with the presence of noodle-like and irregularly shaped precipitates evenly distributed throughout the surface of the alloy. It is important to note that spherical grains accounts more for a large volume of white phase than the black phase. While the noodle-like and irregularly shaped structures comprises mostly of the gray and black phases respectively. No indication of dendritic structures was observed for this alloy. Fig. 1(b and c) on the other hand, reveals the formation of dendritic structure composed of (i) dendritic core (DC) composing of cuboidal precipitates and (ii) interdendritic regions (ID) consisting of columnar structures. It is evident that the increase in aluminium content fosters the formation of dendrites and coarsening of grains. This was also the case in the reported alloys [15]. The formation of DC and ID within the alloy entails that solid-solution phases coexist in the alloy [16]. The addition of Al to 20 at% caused the formation of well pronounced grain boundaries after annealing at 1000 °C. In addition to Fig. 1(b and c), gray and black are representative of main phases since they occupy a larger volume on the surface of the alloys. A significant reduction in the white phase was observed as the Al concentration was increased. The prediction on formation of single phase solid solutions has to meet the proposed criteria where the formation of FCC and BCC develop in the region of X 1.1, d 6.6%, 15 kJ/mol < DHmix < 5 kJ/mol [17]. The VEC criterion is another factor that is to predict the stability of phases. For instance, FCC phase forms when VEC 8.0 and BCC at VEC < 6.87. for two phases to coexist the VEC value has to be between the range 6.87–8.0 [18]. Table 1 contains the cal-
Fig. 1. SEM images of alloys annealed at 1000 °C with Al at% of (a) 10, (b) 15 and (20).
Please cite this article as: K. Masemola, N. Malatji and A. Patricia Popoola, Evaluation of microstructural and mechanical properties of AlxCrFeMnNi high entropy alloys, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.12.320
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Table 1 Empirical values of enthalpy (DHmix) and entropy (DSmix) of mixing, melting point (Tm), atomic size mismatch (d), regular solution-interaction parameter (X) and valence electron concentration (VEC) of the alloys with varying Al content (atm%).
10 15 20
DHmix (kJ/mol) 9.32 10.80 12.48
DSmix (J/Kmol)
Tm (K)
d
X
VEC
13.08 13.31 13.38
1721.88 1678.07 1634.26
5.83 4.76 5.26
2.42 2.06 1.75
7.28 7.03 6.80
culated values of the parameters used to predict the formation of solid solutions. Calculated values show that the conditions favourable for the development of FCC and BCC phases are met. Moreover, sole BCC phase in absence of FCC is expected when x = 20 and a mixture of both phases when x = 15 and 10. These predictions are confirmed by XRD patterns of the experimental alloys. The XRD patterns of the current AlxFeNiMnCr HEAs are shown in Fig. 2. It can be seen that the alloys constitute mainly of bodycentered cubic (BCC) and face-centered (FCC) structures. At x = 10, the sharp peaks of BCC structure depicts the high amount of Cr while the low intensity peaks of FCC indicates the presence of Fe and Ni. The Al10FeNiMnCr alloy displays a larger volume fraction of FCC phase. However, the alloy containing Al content of 15 at % shows a lower volume fraction of FCC and at 20 at% no FCC was identified. It is evident that the increase in Al in the AlxFeNiMnCr HEA decreases the quantity of FCC in the alloys. These results are consistent with what has been reported in literature [9]. The Al10FeNiMnCr alloys displays almost equal distribution of BCC and FCC phase due to a lower amount of Aluminium. Fig. 3 shows the microhardness of the AlxFeNiCrMn high entropy alloys. The increment of Al promoted decrease in the hardness of alloys. With Al content up to 10 at% a comparatively high hardness value of 495 HV was recorded followed by 410 HV with x = 15, and 397 HV at x = 20. This behaviour as reported in other alloys [19], can be attributed to the fact that Al is softer as compared to other elements having higher strength in the alloy. In addition, this confirms that there is no formation of intermetallic phases within the alloys which may harden the alloy. The low hardness value displayed by the equi-atomic HEA may be due to recrystallization caused by excessive annealing temperatures for prolonged periods. Shahmir and others [12] discovered that short term annealing of CoCrFeNiMn alloy at 800 °C for 10 min prevents grain growth and leads to a combination of high strength and good ductility. Compressive properties of the heat treated AlxFeNiCrMn HEAs are shown in Fig. 4. As can be seen, the plasticity of alloys decreases with increasing Al concentration. The highest total elongation of
600 Microhardness (HVN)
Al at% in alloy
495
500
410
397
400
300
10
15 Al at% in alloy
20
Fig. 3. Vickers hardness measurements of heat-treated AlxFeNiMnCr alloys.
around 24% was observed in the alloy containing x = 10. This could be due to the presence of larger volume fraction of FCC phase as compared to alloys consisting x = 15 and x = 20 which is depicted from XRD results. It has been reported that FCC contains more slip directions and therefore readily experiences slip deformation which ultimately lowers the strength and hardness of the material [20]. With Al concentration of x = 10 the ultimate strength is approximately 1200 MPa. However, when x = 15 and x = 20 the obtained ultimate strength was around 1429 and 1222 MPa respectively with similar total elongation of nearly 20%. Adjusting the Al content above 10 at% significantly reduced the plasticity of the alloys. The presence of BCC solid-solution phase exhibits high strength and hardness with poor ductility [21]. The Al15FeNiCrMn displayed a good combination of high strength and ductility which could be attributed to adequate amounts of BCC and FCC phases in the high entropy alloy. Noteworthy, Al addition may improve the mechanical performance of the alloy. However, excessive amounts will be counterproductive. This observation is in agreement with the reported results [22].
Engineering stress (MPa)
1600 1400 1200 1000
x = 20
800
x = 15
600
x = 10
400 200 0
0,00
10,00
20,00
30,00
40,00
Engineering strain (%) Fig. 2. XRD patterns of the annealed AlxFeNiMnCr.
Fig. 4. Compression curves of AlxCrFeNiMn alloys.
Please cite this article as: K. Masemola, N. Malatji and A. Patricia Popoola, Evaluation of microstructural and mechanical properties of AlxCrFeMnNi high entropy alloys, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.12.320
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4. Conclusion The AlxFeNiCrMn HEAs were successfully fabricated via arcmelting and annealed at 1000 °C for 2 h in air. Microstructural evolution, microhardness and compressive strength properties of the alloys under investigation were examined. It is concluded that the increment of Al caused the hardness to decrease in the annealed alloy. An inverse relationship between the compressive strength and total elongation was observed with the addition of Al. An increase in elemental Al fostered a reduction in plasticity and an increase compressive strength. Moreover, the formation of dendritic structures were identified with Al above 10 at%. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements The authors would like to acknowledge the following organizations. Mintek, Advanced Materials Division, Randburg. Tshwane University of Technology, Department of Chemical, Metallurgical Engineering and Materials Sciences, Pretoria, South Africa References [1] A. Munitz, S. Samuh, E. Brosh, S. Salhov, N. Derimow, R. Abbaschian, Liquid phase separation phenomena in Al2.2CrCuFeNi2 HEA, Intermetallics 97 (2018) 77–84. [2] Y. Chen, S. Zhu, X. Wang, B. Yang, G. Han, L. Qiu, Microstructure evolution and strengthening mechanism of Al0.4CoCu0.6NiSix (x¼0e0.2) high entropy alloys prepared by vacuum arc melting and copper injection fast solidification, Vacuum 150 (2018) 84–95. [3] A.J.B. Xi Jin, L. Zhang, Y. Zhou, X. Du, Y. Liang, B. Li, A new CrFeNi2Al eutectic high entropy alloy system with excellent mechanical properties, J. Alloys Compd. 770 (2019) 655–661. [4] Z.G. Zhu, K.H. Ma, X. Yang, C.H. Shek, Annealing effect on the phase stability and mechanical properties of (FeNiCrMn)(100–x)Cox high entropy alloys, J. Alloys Compd. 695 (2017) 2945–2950. [5] C.-C. Yen, H.-N. Lu, M.-H. Tsai, B.-W. Wu, Y.-C. Lo, C.-C. Wang, S.-Y. Chang, S.-K. Yen, Corrosion mechanism of annealed equiatomic AlCoCrFeNi tri-phase highentropy alloy in 0.5 M H2SO4 aerated aqueous solution, Corrosion Sci. (2019).
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Please cite this article as: K. Masemola, N. Malatji and A. Patricia Popoola, Evaluation of microstructural and mechanical properties of AlxCrFeMnNi high entropy alloys, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.12.320